We propose to isolate and characterize of the numerous enzymes and proteins responsible for (A) Initiation of a cycle of replication of the E. coli chromosome at its origin (oriC), and (B) Initiation of nascent chains at the replication forks of the chromosome. Progress with the enzymology of oriC initiation in the past year and with nascent chain initiation and elongation for the past eleven years encourages us to pursue several objectives: (A1) Kinetics of assembly, stoichiometry and morphology of the multicomponent system that recognizes the unique and remarkable 245-basepair consensus sequence of Enterobacteriacea. (A2) Influence of alterations in conserved sequences of basepairs and in frame distances between conserved sequences on bidirectional replication. (A3) Enzymology of plasmids related to oriC (e.g., pSC101) that also depend on dnaA and dnaC proteins. (A4) Mechanisms of RNA polymerase, catenase, gyrase, dnaA protein, novel factors and the known replication proteins. (A5) Positive and negative replication regulatory elements that operate the cellular switch for a cycle of replication. (B1) Kinetics of assembly, stoichiometry and morphology of the multicomponent primosome. (B2) Relationship of priming and helicase functions of the primosome to the oriC system (above) and the DNA polymerase III holoenzyme at the replication fork to operate as a replisome. Progress in these biochemical studies should reveal facts and patterns of importance to the biology of cell growth and the factors which distort orderly growth to cause premature cessation or uncontrolled proliferation. Extension of these findings with a prokaryotic system promises, as in the past, to be of great value with studies of more complex eukaryotic systems.